The following correction was printed in the Guardian's Corrections and clarifications column, Saturday March 24 2007

Errant capitalisation in the phrase "the Right to Roam Act" in the article below gave the impression that this is the proper title of the legislation. It is called the Countryside and Rights of Way Act 2000.

A new form of rural "happy slapping" in which young drivers taunt farmers and film their reaction on mobile phones has been condemned by a judge as "reprehensible".

Tensions in Cornwall have escalated between off-road drivers of cars and motorbikes who claim they have access rights to "common" moorland under the Right to Roam Act, and upland farmers who say "illegal" drivers are abusing the law, killing livestock and threatening the moors' delicate environment.

"There seems to be a new craze of going up on the moor, taunting the farmer, and filming it," said Julie Dowton, secretary to the Commoners Association. "Upland farming is tough enough as it is. But when youths in cars come up here, or motorcyclists chase livestock, it can cause big problems." She said farmers are "facing a volley of abuse and being filmed on a mobile phone".

Last week a farmer was convicted of inconsiderate driving and failing to report an accident following a confrontation with four youths, aged between 17 and 20, who, he alleged, killed scores of animals while racing around a stretch of Bodmin moor.

The jury at Truro crown court heard how Billy Steed, 27, from Davidstow, rammed the youths' car with his 4x4 in April last year after an hour-long chase across Davidstow Common.

He was cleared of the more serious charge of dangerous driving and conditionally discharged by Recorder Richard Stead, who described the behaviour of the group of young men as "reprehensible".

In mobile phone footage taken by the youths and shown to the court by the prosecution, the four are seen shouting abuse at Mr Steed's 67-year-old father, Tony.

The young men, who denied killing livestock, told the court Mr Steed was a "nutcase" and a "psycho" who they had "wound up" for fun.

Other landowners have rallied to his side. "The police cannot marshal remote moorland, and it is difficult to know how you would react if you were being deliberately antagonised on film," said Bodmin landowner Rupert Hanbury-Tenison.